This invention relates to a multi-layered thin film solar cell in which solar light successively enters plural photoelectric conversion layers having different light sensitivities.
For improving the efficiency of a thin film solar cell such as an amorphous silicon type solar cell, effective utilization of the solar light spectrum is indispensable. Since the conversion efficiency is restricted in a thin film solar cell using a single photoelectronic conversion layer, it is necessary to laminate two or more photoelectric conversion layers as shown in FIG. 3 in order to increase the utilization efficiency of the solar light by dividing the sensitivity region to the solar light spectrum. In FIG. 3, for light 10 passing through a light permeable substrate 1 and a transparent electrode 2, the shorter wavelength portion is absorbed by a first photoelectric conversion layer 31 with greater optical band gap (Eg), the longer wavelength portion is absorbed by a third photoelectric conversion layer 33 with smaller optical band gap (Eg), and the medium wavelength portion is absorbed by a second photoelectric conversion layer 32 with an intermediate optical band gap (Eg).
The power of the solar cell having a laminated structure of photoelectric conversion layers of different sensitivity regions is outputted through the transparent electrode 2 and the back electrode 4. It is shown by theoretical calculations that a conversion efficiency of about 20% can be obtained for an amorphous silicon type solar cell and various studies have been made to obtain a multi-layered thin film solar cell.
From a practical point of view, however, the structure shown in FIG. 3, in which a plurality of photoelectric conversion layers are successively laminated on a substrate, has several problems. At first, since each of the photoelectric conversion layers is laminated successively, the device structure has to be designed such that equal electrical currents are generated in each of the photoelectric conversion layers. Further, if the spectrum of the solar light varies depending on the season or site location, design adaptability can no longer be attained and the advantage obtained by adopting the multi-layered structure is reduced due to the non-uniformity of the current. Secondly, since an n-p or p-n junction is formed at the interface between each of the photoelectric conversion layers, recombination loss of carriers occurs or reverse voltage is generated at the junction, causing a reduction in the cell power.
As a countermeasure, a thin film solar cell module as shown in FIG. 4 has been proposed in Japanese Laid Open Pat. No. Sho 60-30163. That is, a group of solar cell units each comprising a laminated transparent electrode 2, a photoelectric conversion layer 31 and a transparent electrode 51 are connected in series on one transparent insulating substrate 1, whereas solar cell units each comprising a metal electrode 4, a photoelectric conversion layer 32 and a transparent electrode 52 are connected in series on the substrate 11. These cell groups are opposed to each other with both of the substrates being on the outside, coupled by way of frames 61 and sealed with transparent resins 62.
Similarly to the case shown in FIG. 3, the optical band gap Eg of the photoelectric conversion layer 31 is greater than that of the photoelectric conversion layer 32. Both of the serially connected solar cells are further connected in parallel with each other by connecting terminals 63 with 64 and terminals 65 with 66, respectively. However, such a module has a drawback that solar cells have to be formed on two substrates separately and the structure is complicated and expensive as well.